Changing the topic, now next, is SNMP
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SNMP Basic Concepts
SNMP Overview
· Simple Network Management Protocol (SNMP) is an application layer protocol widely used in TCP/IP network for collecting, managing and modifying information of managed devices.
· Being the part of TCP/IP protocol suite, the SNMP message are wrapped as User Datagram Protocol (UDP) and transmitted in the Internet Protocol.
SNMP Architecture.
· An SNMP-managed network consists of 3 key components, including the managed device, agent and the Network Management System (NMS).
An SNMP-managed network consists of 3 key components, as per listed below:
1. Managed object: devices or network elements to be monitored.
2. Agent
An agent is the process run on the managed devices. After the managed device receives the request sent by the NM station, the agent is responsible for responding to the request.
The agent has the following functions:
Collects the information about device status.
Supports remote operations on the device through NMS.
Sends trap messages to the NM station.
3. NMS
Network Management System (NMS) is the network management software run on the Network Management station (NM station). The network manager sends requests to the managed devices and monitors and configures the network devices through NMS.
In SNMP, the NMS and the agent communicate through packet exchanging.
The NMS acts as a manager and sends an SNMP request packet to the SNMP agent.
The agent searches in the MIB on the device for the required information and sends a response packet to the NMS.
The agent sends a trap message to the NMS when the value of some module on the managed device exceeds the defined threshold. According to the trap message, the network manager can process the occurred event in time.
SNMP Protocol Framework
To simplify the development of the Agent side, SNMP only defines two kinds of operations --- Getand Set. Get is used to obtain management information from managed equipment. And Set is used to configure managed equipment via setting the value of variable.
NMS and Agent transfer management information to each other via packet.
And SNMP V1 only defines five kinds of packets:
--Get Request packet: Used to get the value of specified management variable.
--GetNext Request packet: Used to continuously get the values of a group of variables.
--GetResponse packet: Used to respond request, return value for request or error type, etc.
--Set Request packet: Used to set the specified management variable.
--Trap packet: Used for managed equipment to send information to NMS initiatively in urgent cases.
GetRequest and GetNextRequest are used to obtain information of the managed object in NM.
SetRequest is used to configure the managed object. These three kinds of requests correspond with three kinds of SNMP messages.
Agent responds them via sending GetResponse message.
Trap is generated by Agent. It is used to report abnormal event of the managed equipment to the NM.
Agent will send Trap to notify NM when equipment gives alarm or important data is changed by user/console/other NMs.
When SNMP Manager receives the Trap, relevant actions will be initiated, such as diagnosing fault via polling, adopting recovery measures, modifying relevant database of the NM.
SNMP Operation Process.
SNMP request and response message operation are described in details as shown below:
1. The agent receives an SNMP request packet from the NMS through UDP port 161.
2. The agent decodes the packet based on ASN.1 basic coding rules and represents it in an internal data structure. The agent discards the packet if there is a decoding failure.
3. The agent gets the version number from the packet. The agent discards the packet if the version is inconsistent with the SNMP version it supports.
4. The agent gets the community name from the packet. The community name is filled by the NMS that sends the request. If the community name is inconsistent with that of the agent, the packet is discarded. A trap message or an Inform packet is generated simultaneously.
5. The agent gets PDUs from the authenticated ASN.1 object. If failed to get the PDUs, the agent discards the packet; otherwise, the agent processes the PDUs.
6. The agent processes PDUs differently and gets the management variables of the corresponding protocol modules by searching nodes that correspond to management variables in the MIB.
7. The agent encapsulates the values of management variables in a PDU, uses the source IP address and port of the request packet as the destination IP address and port, and adds the SNMP version number. A response packet is then generated. After being coded, the response packet is sent to the NMS
Trap is an unprompted behavior of the managed device. It does not belong to the basic operations on the NMS.
As shown in the figure on the right above, if the value of some module exceeds the defined threshold, the agent sends a trap message to the NMS.
The NMS receives the trap message from UDP port 162, according to which the manager can process the network abnormity in time.
In the case that the interface status changes, the agent also sends a trap message to the NMS. The
manager can then diagnose and rectify the fault according to the trap message.
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